Die cooling apparatus and method thereof

ABSTRACT

A die cooling system for cooling a cast wheel for an automotive vehicle is provided that includes a die, multiple thermocouples embedded in the die, which measure the actual cast metal temperature, adjustable cooling valves, and a control system. The control system receives actual casting metal temperature data from each thermocouple to control the operation of the adjustable cooling valves.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and method for cooling adie casting product and more specifically to an apparatus and method formeasuring the actual temperature of the die cast product and controllingthe solidification rate of the die cast product based on the actualtemperature of the die cast product.

2. Description of Related Art

Low pressure die casting methods and procedures to produce cast productsare well known in the industry. Basically, the low pressure die castingmethod comprises a metal die mounted above a sealed furnace thatcontains molten metal. A refractory lined tube, called a stalk tube,extends from the bottom of the die into the molten metal. Air or a gasunder low pressure is introduced into the furnace. The air or gas forcesthe molten metal up the tube and into the die cavity. When the metalinside the die has solidified the pressure in the furnace is releasedand the molten metal in the tube returns to the furnace. After anadditional cooling time the die is opened and the casting is extracted.

Specific methods and procedures during this process can significantlyalter the resulting end product. For example, the cooling rate and hencethe solidification rate at which the molten metal solidifies can affectthe microstructure of the finished product. More specifically, a fastsolidification rate produces a fine microstructure, which leads to goodelongation properties. If the solidification rate is too fast, however,shrinkage-porosity may develop in the end product. On the other hand, aslow solidification rate results in unacceptable mechanical properties.More specifically, a slow solidification rate creates a coarsemicrostructure in the material, which in turn creates poor elongationproperties.

Conventional methods to control the cooling rate and ultimately thesolidification rate of the cast product include using thermocouples tomeasure the temperature of the die, referred to as an in-diethermocouple system, and using this information to control on/off typecooling valves, which blow cooling air directly onto the die. Onedisadvantage to the in-die thermocouple system is that the in-diethermocouple system can be affected by variations in atmospheretemperature, die coating thickness, die coating thermal diffusion, diecooling air temperature and humidity, and metal temperaturefluctuations. The present invention can adjust to these variations,which results in more uniform properties throughout the entire castproduct than in the in-die thermocouple system. This in turn reduces theamount of scrap, which is yet another advantage of the present inventionover conventional methods.

In today's the automotive industry, however, the trend is to manufacturelarger diameter wheels for aggressive styling. In 2001 the typical wheeldiameter on North America automotive vehicles ranged from 14 inches to17 inches. As of 2005, while the majority of the wheel diameters stillranged between 15 inches to 17 inches, nearly 10% of the wheel diameterswere greater than or equal to 18 inches. An increase in the wheeldiameter, however, introduced problems in the casting process. Morespecifically, conventional methods. Like those mentioned above, tocontrol the cooling rate are no longer efficient enough to achieve theproper mechanical properties in all areas of the wheel and morespecifically in the spoke portions of the wheel. Thus, what is requiredis a die cooling apparatus and method to more uniformly cool a castautomotive wheel.

SUMMARY OF THE INVENTION

In accordance with one aspect, the present invention overcomes the abovementioned disadvantages by providing a die cooling system for coolingcast metal comprising a die, multiple thermocouples embedded in the die,adjustable cooling valves, and a feedback control system. Thethermocouples are embedded in the die and contact the cast metal andmeasure the actual casting metal temperature. The control systemreceives measured temperature signals from each thermocouple to therebycontrol the operation of each adjustable cooling valve to maintain theproper solidification rate of the casting metal.

Additional benefits and advantages of the present invention will becomeapparent to those skilled in the art to which it pertains upon a readingand understanding of the following detailed specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take physical form in certain parts and arrangement ofparts, a preferred embodiment of which will be described in detail inthis specification and illustrated in the accompanying drawings thatform a part of the specification.

FIG. 1 shows a die cooling system in accordance with the presentinvention.

FIG. 2 is a view of a spoke portion of the die.

FIG. 3 is a view taken along line 3-3 of FIG. 2.

FIG. 4 is a graph showing a die cooling graph.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, FIG. 1 shows a die cooling system 10 forcooling a low-pressure die for forming large die cast wheels. The diecooling system 10 includes a die 12, adjustable cooling valves 14,multiple thermocouples (TC1 . . . TCN), and a feedback control system16. The die cooling system 10 overcomes the above mentioneddisadvantages by systematically controlling the operation of theadjustable cooling valves 14 to maintain a solidification rate of thecasting metal during the cooling process to produce optimum mechanicalproperties. It should be noted that although the die cooling systemdisclosed herein is designed to improve the casting process for wheelswith a diameter greater than or equal 18″, the die cooling system 10 canbe implemented for casting wheels having a diameter less than 18″.

For simplicity and illustrative purposes only, the present inventionwill be described with reference to one spoke portion of the die 12, asshown in FIGS. 2 and 3. It should be noted, however, that each spokeportion of the die 21 has the same cooling arrangement as will besubsequently described.

Referring to FIGS. 2 and 3, the die 12 is a conventional die of the typecommonly known in the art that includes a top core 18, an inner-sidecore 20, an outer-side core 22 and a bottom core 24. The die 12 furtherincludes a conventional cooling air delivery structure (not shown) ofthe type commonly known in the art and will not be described herein.Thus, the adjustable cooling valves 14, shown schematically in FIG. 3,deliver cooling air to the die 12 via the delivery structure. It shouldbe noted that the number of adjustable cooling valves 14 shown in FIG. 3is for illustrative purposes only and is not intended to limit the scopeof the invention. Thus, the number of adjustable cooling valves 14 mayvary depending on the size of the die 12 the material of the castproduct, etc.

Referring to FIG. 4, the thermocouples TC are embedded into the die 12such that a tip 26 of each thermocouple TC is either flush with aninside surface 28 of the die 12 or slightly embedded into the castingmetal. Thus, each thermocouple TC contacts the casting metal and,therefore, measures the actual casting metal temperature and not the dietemperature. Because the thermocouples TC measure the actual castingmetal temperature, the die cooling system 10 ensures that casting metalsolidifies at the solidification rate, as explained further below. Itshould be noted that the number of thermocouples TC may vary dependingon the diameter of the wheel being cast. For example, a wheel having adiameter of 18″ may require fewer thermocouples than a wheel having adiameter of 20″. In addition, because the casting metal will solidify atthe same rate from either side of the die 12, it is contemplated thatthermocouples TC may be required on only one side of the die.

A release agent may be applied to the inside surface 28 of the die 12 tofacilitate the removal of the cast product when completed. Prior toapplying the release agent the thermocouples TC are masked off so therelease agent does not hinder the performance of the thermocouples TC.

Referring to FIG. 1, the feedback control system 16 includes a computer30 and a programmable controller 32. The computer 30 is electronicallylinked to each thermocouple TC and continuously receives signalsindicative of the actual casting metal temperature (temperature data)from each thermocouple TC. The computer 30 transmits the temperaturedata to the programmable controller 32. In order to achieve and maintainthe solidification rate, the programmable controller 32 processes thetemperature data to control the operation of the adjustable coolingvalves 14. In addition, the programmable controller 32 communicates thestatus of each adjustable cooling valve 14 back to the computer 30.

FIG. 4 illustrates an example embodiment of a die cooling graphcomprising a combination bar graph and line graph. The bar graphrepresents a cooling zone diagram and illustrates how the die 12 iscooled. The line graph, referred to as metal cooling curves, representsthe temperature of the casting metal collected by each thermocouple TC.As an option, the cooling zone diagram and the metal cooling curves canbe displayed on a monitor (not shown) connected to the computer 14. Itshould be noted that the cooling zone diagram and the metal coolingcurves shown in FIG. 4 is just one example of a cooling a die and is forillustrative purposes only and is not intended to limit the scope of theinvention.

Referring to the cooling zone diagram (bar graph) of FIG. 4, a coolingzone CZ is defined as a location within the die cooling system 10 wherecooling air is applied to the die 12. The cooling zones CZ areschematically represented by the adjustable cooling valves 14, as shownin FIG. 3. Thus, the cooling air applied to the die 12 at the coolingzones CZ is supplied by the adjustable cooling valves 14 via cooling airdelivery structure and is regulated by the programmable controller 32.As explained below, cooling air is supplied to the cooling zones CZ in amanner such that the die 12 is systematically cooled from the outer mostportion of the die 12 toward the inner most portion of the die 12. Itshould be noted that the operation of the adjustable cooling valves 14,and hence, the cooling zones CZ operate independently of each other. Forexample, the time at which the cooling zone adjustable cooling valve 14is activated, the duration of its activation and it subsequentdeactivation depends on the casting parameters of the wheel being castexplained further below and the temperature of the casting metal and notnecessarily on the status of the previous cooling zone CZ. In addition,activation of adjustable cooling valves 14 for adjacent cooling zones CZmay or may not overlap. Still further, the amount of overlap may vary.

Referring to the line graph of FIG. 4, as mentioned above, the metalcooling curves represent the actual casting metal temperature. Forexample, the metal cooling curve denoted as TC1 is the actual castingmetal temperature as measured by thermocouple TC1, the metal coolingcurve denoted as TC2 is the actual casting metal temperature as measuredby thermocouple TC2, etc. As illustrated by the metal cooling curves thecasting metal is a liquid above a first temperature and becomes a solidbelow a second temperature. As the casting metal cools from the firsttemperature down to the second temperature the casting metal is in asemi-solid state and undergoes a transformation from a liquid to asolid. This applies to any metal that has a transformation temperature,such as, for example but not limited to aluminum. This transformationoccurs over a time period referred to as a solidification period. Therate at which the casting metal transforms from a liquid to a solid isthe solidification rate mentioned above. Maintaining the propersolidification rate, referred to as a critical solidification rate,during the solidification period is critical in order to achieve optimummechanical properties of the casting metal.

For example, if the casting is not cooled at the critical solidificationrate then either macro-shrinkage porosity or micro-shrinkage porositymay occur within the casting. More specifically, if a portion of thecasting is cooled too early or too aggressive then the molten metal feedfrom the gate will be cut off. This in turn causes macro-shrinkageporosity between portions of the casting, such as for example, betweenthe spoke and the flange. Micro-shrinkage porosity can occur in a morelocalized area if the solidification rate falls below the criticalsolidification rate. More specifically, if the solidification rate fallsbelow the critical solidification rate then the dendrite arms become toolong. This leads to micro-shrinkage porosity between the dendrite arms.

Prior to operation of the die cooling system 10 the feedback controlsystem 16 calculates the critical solidification rate of the castingmetal at each thermocouple TC location. The critical solidification rateis calculated by taking into account certain casting parameters for aparticular cast wheel, such as the temperature of the casting metalduring the casting process, the diameter, width, thickness, design,material, etc. of the wheel, etc. The casting parameters are enteredinto the feedback control system 16 whereby the feedback control system16 calculates the critical solidification rate for the cast wheel. Itshould be noted that the critical solidification rate of the castingmetal may vary for each thermocouple TC location due to characteristicsof the wheel, such as thickness.

During operation of the die cooling system 10, molten metal isintroduced into the die 12 using the low-pressure die casting methoddescribed above. The computer 30 continuously processes temperature datafrom each thermocouple TC and transmits this information to theprogrammable controller 32. The programmable controller 32 uses thetemperature data to control the operation of the adjustable coolingvalves 14 for each cooling zone CZ accordingly. In addition, theprogrammable controller 32 uses the temperature data from a giventhermocouple TC to control the operation of a downstream adjustablecooling valve 14. For example, the programmable controller 32 cancontrol the operation of the adjustable cooling valves 14 downstreamfrom thermocouple TC1 based on the temperature data received fromthermocouple TC1. Thus, based on the temperature data the programmablecontroller 32 activates, deactivates and adjusts the flow rate of theadjustable cooling valves 14 such that cooling air is systematicallysupplied to the cooling zones CZ from the outer most portion of the die12 toward the inner most portion of the die 12 to thereby maintain thecritical solidification rate. As shown in the bar graph of FIG. 4, thepresence of a bar indicates that an adjustable cooling valve 14 isactivated and is, thus, supplying cooling air to the correspondingcooling zone CZ. It should be noted that when an adjustable coolingvalve 14 is activated the programmable controller 32 may also regulatethe amount of air flow from each adjustable cooling valve 14. Forexample, the adjustable cooling valves 14 may be variably opened between0% and 100%. Thus, based on the temperature data from the thermocouplesTC the programmable controller 32 can regulate the on/off operation ofthe adjustable cooling valves 14 or vary the flow rate of the adjustablecooling valves 14.

As an example, referring to the embodiment shown in FIG. 4, as thecooling process begins the casting metal begins to cool gradually asshown by the metal cooling curve represented by thermocouple TC1. Theprogrammable controller 32 activates the adjustable cooling valve 14 forcooling zone CZ1 and the casting metal represented by metal coolingcurve TC1 begins to cool more rapidly. As the casting metal enters andproceeds through the transformation phase described above, theprogrammable controller 32 may also adjust the flow rate of theadjustable cooling valve 14 to thereby more precisely maintain thecritical solidification rate. When the feedback control system 16determines that the transformation has successfully occurred or willsuccessfully occur at cooling zone CZ1 the programmable controller 32will either deactivate the adjustable cooling valve 14 for cooling CZ1or adjust the flow rate of the adjustable cooling valve 14. This processcontinues through each cooling zone CZ such that the casting metalsystematically solidifies from the outer most portion of the die 12 (thewheel flange 34) toward the inner most portion of the die 12 closest tothe stalk tube. Once the entire casting metal is transformed from aliquid to a solid the die 12 is quickly cooled to a temperature suchthat the cast wheel can be extracted from the die 12.

The die cooling system 10 embodiment described above is capable ofcooling casting metal at a critical solidification rate calculated fromcasting parameters to thereby ensure that the casting metal solidifieswith optimum mechanical properties. This example embodiment employs acombination time-temperature based method to control the operation ofthe adjustable cooling valves 14. Thus, the feedback control system 16controls the operation of the adjustable cooling valves 14 based on thetemperature data from the thermocouples TC and on the solidificationrate of the cast metal.

It is contemplated, however, that a time based method may be employed tocontrol the operation of the adjustable cooling valves 14. For example,the adjustable cooling valves 14 can be activated, deactivated, orvaried based on the based on the critical solidification rate of thecast metal.

It is further contemplated that a temperature based method may beemployed to control the operation the adjustable cooling valves 14. Forexample, the adjustable cooling valves 14 can be activated, deactivated,or varied based on the temperature data received by the feedback controlsystem 16 from the thermocouples TC. Thus, the cooling air flow from theadjustable cooling valves 14 at each cooling zone CZ can be regulatedbased on the temperature of the casting metal.

It is still further contemplated that the adjustable cooling valves 14for each cooling zone CZ may be all activated once the cooling processbegins and that the programmable controller 32 varies the flow rate ofeach adjustable cooling valve 14 accordingly to achieve and maintain thecritical solidification rate.

While specific embodiments of the invention have been described andillustrated, it is to be understood that these embodiments are providedby way of example only and that the invention is not to be construed asbeing limited but only by proper scope of the following claims.

1. A die cooling system for cooling cast metal comprising: a die; aplurality of thermocouples embedded in the die, such that eachthermocouple contacts the casting metal; a plurality of cooling zonesarranged such that cooling air is supplied to the cooling zones to coolthe die; and a feedback control system, wherein each thermocouplemeasures the temperature of the cast metal, and wherein the controlsystem continuously receives temperature data from each thermocouple andcontrols the supply of cooling air to the cooling zones based on thetemperature data.
 2. The die cooling system of claim 1, wherein a tip ofeach thermocouple is flush with an inside surface of the die.
 3. The diecooling system of claim 2 further comprising a plurality of adjustablecooling valves to supply cooling air to the cooling zones, wherein thecontrol system calculates a critical solidification rate of the castmetal and activates and deactivates the adjustable cooling valves tosupply cooling air to corresponding cooling zones in such a manner thatthe die is gradually cooled at the critical solidification rate from anouter most portion of the die toward an inner most portion of the die.4. The die cooling system of claim 3, wherein during activation of theadjustable cooling valve the control system varies the cooling air flowrate of the adjustable cooling valve to maintain the criticalsolidification rate of the cast metal.
 5. The die cooling system ofclaim 1, wherein a tip of each thermocouple is embedded in the castingmetal.
 6. The die cooling system of claim 5 further comprising aplurality of adjustable cooling valves to supply cooling air to thecooling zones, wherein the control system calculates a criticalsolidification rate of the cast metal and activates and deactivates theadjustable cooling valves to supply cooling air to corresponding coolingzones in such a manner that the die is gradually cooled at the criticalsolidification rate from an outer most portion of the die toward aninner most portion of the die.
 7. The die cooling system of claim 6,wherein during activation of the adjustable cooling valve the controlsystem varies the cooling air flow rate of the adjustable cooling valveto maintain the critical solidification rate of the cast metal.
 8. A diecooling system for cooling a cast wheel for an automotive vehiclecomprising: a die; a plurality of thermocouples embedded in the die suchthat each thermocouple contacts the cast wheel; a plurality adjustablecooling valves to supply cooling air to various portions of the die; anda feedback control system, wherein each thermocouple measures the actualtemperature of the cast wheel, and wherein the control systemcontinuously receives temperature data from each thermocouple andcontrols the operation of the adjustable cooling valves based on thetemperature data.
 9. The die cooling system of claim 8, wherein a tip ofeach thermocouple is flush with an inside surface of the die.
 10. Thedie cooling system of claim 9, wherein the control system calculates acritical solidification rate of the cast wheel, wherein the controlsystem includes a computer and a programmable controller, wherein thecomputer receives temperature data from each thermocouple and transmitsthe temperature data to the programmable controller, and wherein theprogrammable controller activates and deactivates the adjustable coolingvalves to supply cooling air to corresponding cooling zones in such amanner that the die is gradually cooled at the critical solidificationrate from an outer most portion of the die toward an inner most portionof the die.
 11. The die cooling system of claim 10, wherein duringactivation of the adjustable cooling valve the control system varies thecooling air flow rate of the adjustable cooling valve to maintain thecritical solidification rate of the cast wheel.
 12. The die coolingsystem of claim 8, wherein a tip of each thermocouple is embedded in thecast wheel.
 13. The die cooling system of claim 12, wherein the controlsystem calculates a critical solidification rate of the cast wheel,wherein the control system includes a computer and a programmablecontroller, wherein the computer receives temperature data from eachthermocouple and transmits the temperature data to the programmablecontroller, and wherein the programmable controller activates anddeactivates the adjustable cooling valves to supply cooling air tocorresponding cooling zones in such a manner that the die is graduallycooled at the critical solidification rate from an outer most portion ofthe die toward an inner most portion of the die.
 14. The die coolingsystem of claim 13, wherein during activation of the adjustable coolingvalve the control system varies the cooling air flow rate of theadjustable cooling valve to maintain the critical solidification rate ofthe cast wheel.
 15. A method of cooling a cast wheel for an automotivevehicle comprising the steps of: providing a die having a plurality ofthermocouples embedded in the die such that each thermocouple contactsthe cast wheel, a plurality of adjustable cooling valves to supplycooling air to the die, and a control system; calculating a criticalsolidification rate of the cast wheel based on parameters of the wheel;filling a cavity of the die with molten metal; measuring the cast wheeltemperature continuously at each thermocouple location; and controllingthe supply of cooling air from the adjustable cooling valves to achievethe critical solidification rate during a solidification period.
 16. Themethod of claim 15 further comprising the steps of: activating anddeactivating the adjustable cooling valves to supply cooling air to eachcooling zone to gradually cool the die from an outer most portion of thedie toward an inner most portion of the die; and varying the cooling airflow rate of an activated adjustable cooling valve to maintain thecritical solidification rate.
 17. The method of claim 16 furthercomprising the steps of: transforming the molten metal from a liquid toa solid; and cooling the entire die rapidly to a temperature such thatthe cast wheel can be extracted from the die.